Current work in vertebrates has established the sufficiency of BMP signaling pathway components to induce ventral cell fates as part of an evolutionarily conserved dorsal-ventral (D-V) patterning mechanism. However, the endogenous roles of individual components of this increasingly complex pathway remain to be determined. In the zebrafish, recently isolated mutants that affect D-V pattern formation can be employed to address this problem. One of the most interesting of these mutants is piggytail (pgy). The five alleles of pgy can be ordered into a phenotypic series, the strongest of which displays a dominant maternal/zygotic phenotype. This allelic series and maternal effect indicate an early strict requirement for pgy function in a graded process that establishes ventral cell identities. Genetic evidence indicates that pgy functions within, or in concert with, a BMP signaling pathway. To define the role of pgy in D-V patterning, this proposal contains the following specific aims. 1) Determine the chromosomal position of pgy by linkage to identified PCR-based genomic markers. 2) Determine the function of pgy relative to identified BMP signaling components by expressing them in mutant embryos and assaying for rescue of the pgy mutant phenotype. 3) Determine the complete loss-of-function pgy phenotype by elimination of maternally contributed pgy to reveal its full role in D-V pattern formation. 4) Determine the cell autonomy of pgy. Transplantation of cells between mutant and wild-type embryos will demonstrate which cells require pgy and the role of pgy relative to intracellular or intercellular signal transduction. 5) Determine the molecular nature of pgy. Cloning of the pgy locus is essential to understanding its function and will allow direct molecular and biochemical tests of the role of pgy . Determining the endogenous role of pgy will lead to further understanding of the mechanism by which BMP signaling pathways function in D-V pattern formation and the regulation of TGF-beta signaling in general.